Flame Stabilization Mechanism Research Articles

Thickened Flame Model Extension for Dual Gas Turbine Combustion: Validation Against Single Cup Atmospheric Test

Abstract The development of predictive combustion models is more and more strategic in the design definition of gas turbine (GT) combustor. The thickened flame model (TFM), despite its high computational cost, is one of the most accurate approach available in literature since it can naturally take into account the nonequilibrium effects into the flame brush (i.e., strain and heat losses) as well as preferential diffusion when hydrogen is employed. Conversely, the original formulation of this combustion model needs several adjustments to accommodate the properties of the mixture when different streams of fuels and/or oxidizers are present in the system. The present work represents a first step in the extension of this combustion model to handle multiple streams of fuels and oxidizers. More specifically, an industrial burner fed with two different fuel streams and air as oxidizer is considered. The pilot fuel line is fed with microhydrogen injections with the aim to enhance the lean blow-out margin, while the main one is with pure methane. Dedicated tests are performed at the Technology for High Temperature laboratory (University of Florence) to retrieve the main information characterizing the burner (emissions, temperature, and pressure pulsations) as well as OH* chemiluminescence for the flame shape and position at the same operating conditions. The comparison between the numerical results and the experimental data will provide highlights about the ability of the extended-TFM to capture the main features of the flame stabilization mechanisms.

Numerical investigation on influence of different geometrical cavity flame holders and flame stabilization mechanism in hydrogen fueled multi-strut-based scramjet combustor

A detailed computational study was carried out to explore how the presence of a cavity and cavity angles influence the performance and combustion mechanisms within a scramjet combustor. This research utilized ANSYS Fluent 2021 to create hexahedral meshes and simulate the complex fluid dynamics in scramjet engines equipped with dual strut injectors. The simulations addressed various critical aspects of fluid mechanics, including the continuity, momentum (via the Navier-Stokes equations), and energy equations. These equations were used to model non-reacting flows, while the species transport equations were applied to analyse reacting flows. The turbulence within the combustor was modelled using the standard K-ε model, a common choice for such high-speed aerodynamic evaluations. The study's primary goal was to assess the effects of different geometrical configurations of cavity flame holders on the performance characteristics of an H2-based scramjet combustor overall efficiency, focusing on mixing efficiency, combustion performance, and total pressure loss. Both configurations with and without a cavity were examined. Key findings indicate that incorporating a cavity into the combustor design leads to developing a robust recirculation zone within the cavity area. This recirculation zone is pivotal in enhancing fuel-air mixing and combustion efficiency, with cavity-based combustors showing an earlier onset of combustion and achieving peak combustion efficiencies around 90–95%. The extent of the recirculation region is notably influenced by the proximity of the strut injector to the cavity's length. Additionally, the presence of a shear layer within the cavity generates a weak shock at the cavity's trailing edge. This shock interaction can adversely affect scramjet combustor performance, especially at higher cavity angles (α) and specific geometric configurations, such as an L/D ratio of 4 and α=30°. The L/D ratio of 4 and α=30° combustor model results in a 13.8% increase in turbulence intensity, which raises air-fuel mixing and there by improves mixing and combustion efficiency by 14.23% and 13.34%, compared to the other depth of the cavities. This advantage is critical, especially considering the compact length of the combustor, which is a desirable attribute in scramjet design.

Role of secondary hydrogen injection on flame stabilization of ammonia/air swirling flames

Ammonia is widely recognized as one of the most advanced hydrogen carriers and can be utilized as a fuel in gas turbines. Premixing hydrogen into the ammonia carbon free fuel system can substantially enhance stable limits, but it significantly promotes cross-reactions, leading to increased NO production. Recent findings related to other fuels suggest that the alternative approach for mitigating combustion instabilities involves introducing minimal pure hydrogen at the chamber inlet in a non-premixed mode. This may introduce a novel approach to stabilize ammonia/air swirl flames by extending the stability through a minimal hydrogen via secondary injection, with minimal impact on increasing nitrogen oxide emissions. In the present study, the flame stabilization mechanism and nitrogen oxide emission behaviors of swirl ammonia/air flames by a secondary hydrogen injection were compared with the premixed ammonia/ hydrogen/air flames. OH-/NO-PLIF and PIV techniques were applied to reveal the lean blow-off characteristics and the reacting flow features. The NOx emissions were measured by the FTIR gas analyzer. The large eddy simulation method with a developed dynamic thickened flame model was employed to further reveal the experimental findings. Experimental results show that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. The secondary hydrogen injection exhibits the stronger enhancement ability for the lean/rich blow-off limits, primarily due to the local diffusion hydrogen flame at the flame root providing more active radicals and higher temperature gases. The NO emission shows an increase with the addition of premixed hydrogen, while in the secondary hydrogen injection, NO emission deteriorates due to higher temperatures, with the NO emission increasing by less than 10% compared with the ammonia flame. In the large eddy simulation analyses, the physical effects of the secondary hydrogen injection enhance the flame stability by increasing the resistance of the flame root to extinction. The heat release rate and the mass fractions of NH and H near the flame root are significantly increased by 2% secondary hydrogen injection. The enhancement of the ammonia decomposition process is stronger with secondary hydrogen injection than with fully premixed hydrogen addition. For 2% secondary hydrogen injection case, the local hydrogen injection to form a non-premixed combustion mode can provide much higher capability to increase the local heat release rate compared to the fully premixed mode.Novelty and significance statement: Introducing small amount of hydrogen to ammonia flame can effectively improving the flame stabilization in a swirl combustor. In this study, it is found that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. Comparing the way of premixing hydrogen in the unburned mixture, a larger effect on blow-off limits extension was found when introducing a separate non-premixed hydrogen at the chamber inlet, which is mentioned as the secondary hydrogen injection. The lean blow-off limits can be extended from about 0.67 to 0.59 when only introducing 2% secondary hydrogen injection by heat value. The NO emission shows in the secondary hydrogen injection, NO emission deteriorates by less than 10% compared with the ammonia flame. A thickened flame model for non-premixed combustion was established and verified adequate with velocity field and flame structure. The mechanisms of enhancing flame stabilization by hydrogen introduction including physical and chemical effects for premixing and injection cases were revealed. Furthermore, the difference in the mechanisms of the two cases was emphasized.

Experimental study on flow field and combustion characteristics of V-gutter and integrated flameholders

Abstract To optimize the integrated flameholder, PIV was used to study flow fields of V-gutter and integrated flameholder under both non-reacting and reacting conditions. PLIF, high-speed cameras, and TDLAS were adopted to capture OH distribution, flame structure, and temperature distribution. Comparative analysis of flow fields, combustion characteristics and flame stabilization mechanisms were analyzed. Results show that heat release increases adverse pressure gradient, which can enlarge the recirculation zone size and recirculation rate compared to non-reacting flow field. The flames of both flameholders exhibit symmetrical structures distributed near the shear layers. The blockage ratio dominates the non-reacting flow field, while the expansion angle dominates the reacting flow field, which can further increase the adverse pressure gradient under reacting condition. The V-gutter flameholder demonstrates better fuel/air mixing and larger recirculation than the integrated flameholder. The combustion performance of the integrated flameholder is inferior to the V-gutter flameholder, albeit with better flow resistance properties.

Evaluation of hydrogen/ammonia substitute fuel mixtures for methane: Effect of differential diffusion

One strategy for utilizing ammonia as an energy vector is to replace conventional hydrocarbons with hydrogen-enriched ammonia in existing or retrofitted combustors. However, the strong differential diffusion effect of hydrogen can significantly alter the combustion properties and cause challenges in combustor operability. Therefore, this numerical study performs a systematic investigation on the effect of differential diffusion on fundamental combustion properties of ammonia/hydrogen fuel blends. The investigated combustion properties span from the initiation of combustion, i.e., ignition, to stretched flame propagation and ultimately to flame stabilization mechanisms. The objective is to evaluate the feasibility of hydrogen/ammonia/air mixtures as substitutes for methane/air particularly considering the implications of differential diffusion effects. To this end, four fuel blends with similar unstretched burning properties are selected: fuel lean ammonia/hydrogen (AH-L), fuel rich ammonia/hydrogen (AH-R), fuel lean methane (M-L) and fuel lean methane/hydrogen (MH-L). First, the forced ignition and stretched flame propagation are evaluated. It is found that the AH-L (AH-R) mixture has a large negative (positive) Markstein length, and thereby among the selected fuel blends, AH-L (AH-R) has the lowest (highest) minimum ignition energy. Then flame stabilization mechanisms are investigated. For a stable flame, AH-L (AH-R) has an intensified (weakened) flame base and a weak (strong) flame tip, which shows a higher flashback (blow-off) propensity. Based on the critical gradient theory, a novel stability regime diagram is proposed. With the regime diagram, the flame stabilization limits (central flashback, boundary layer flashback, stable, and blow-off) and flow conditions can be determined for burners of different sizes.

LES investigation of a swirl stabilized technically premixed hydrogen flame with FGM and TFM models

Climate change due to carbon emissions is one of the aspects that must be kept under control nowadays. In this fashion, the employment of hydrogen as fuel provides a fundamental solution for both power generation and transportation since lean premixed combustions allow to reset the carbon emissions and reduce the pollutant ones. Indeed, the higher reactivity of hydrogen permits to operate with a very low equivalence ratio which can drastically limit the NOx production with respect to the common fossil fuels. On the other hand, lean hydrogen mixtures are characterized by peculiar aspects that make their study through CFD simulations not straightforward. The present work aims to analyze through two different numerical approaches a technically premixed hydrogen flame experimentally investigated at the Technische Universität Berlin (TUB). In particular, the main differences between the Flamelet Generated Manifold and the Thickened Flame Model strategy are deeply analyzed as well as the effects of the thermal boundary conditions on the flame stabilization mechanism. The comparison with the experimental data shows the treatment employed drastically influences the accuracy of the obtained outcomes.

Performance study of a central reentrant step pilot-flameholder in a dual-mode combustor

The augmented/ramjet combustion chambers are characterized by wide operating ranges, large flow state variations, and harsh operating conditions. The reliable ignition of these chambers urgently needs to be achieved. To improve the pilot ignition and blowout performance based on the centerbody in augmented/ramjet combustion chambers, a reentrant step flameholder is proposed on the basis of a conventional backward step. In this paper, the flow characteristics, ignition and blowout boundary under the non-premixed inflow, dynamic ignition process and flame stabilization mechanism of the reentrant step flame holder are investigated by using an experimental method with numerical simulation. The results show that the reentrant step flame holder exhibits better ignition and blowout performance than the conventional step flame holder without increasing the flow resistance. The ignition and blowout equivalence ratios of the reentrant step flame holder decrease by 22.86 % and 24.34 %, respectively, compared with those of the conventional step flame holder. The radial reentrant expansion is more effective than the axial reentrant expansion. Analyzing different reentrant structures shows that radial expansion can shorten the ignition delay time and improve the time-averaged integral flame intensity; axial expansion can improve the time-averaged integral flame intensity and reduce the flame pulsation rate; and the structure Da30Dr30 (the conventional step structure expands axially and radially by 30 mm simultaneously within the centerbody.) whose pilot flames are all stabilized in front of the reentrant step flame holder under different incoming flow conditions, exhibits the best ignition and blowout performances.

Influence of velocity on the transient auto-ignition of ethylene jet in a hot coflow

The difficulty of ignition is one of the main problems in the combustion chamber of the air-breathing combined engine. Based on this, the transient auto-ignition in the turbulent non-premixed flames of ethylene in a high-speed jet in hot coflow was studied using a newly developed burner. High-speed hydroxyl (OH) planar laser-induced fluorescence (up to 10 kHz) and large eddy simulations (LES) were performed to investigate the auto-ignition process. The experimental conditions included injection velocities of 334–491 m/s, and the effect of jet velocity on the mixing process, ignition process, and flame stabilization mechanisms were investigated. The results showed that the numerical simulation results were consistent with the experimental data. The velocity could affect the auto-ignition position, whereas it had no effect on the auto-ignition time. The increase in fuel jet velocity is beneficial to the mixing of fuel and high-temperature gas and helps to increase the premixed combustion zone during the formation of fire kernels. Moreover, as the velocity increased, the flame status changed from a transitional flame to a lifted flame, the growth rates of the flame volume and total heat release increased, and the flame stabilization mechanism changed from flame propagation to auto-ignition.

The influence of clustering in homogeneous isotropic turbulence on the ignition behavior of iron particles

We conduct carrier-phase direct numerical simulations (CP-DNS) to investigate the ignition of iron particles in homogeneous isotropic turbulence, and characterize the connection between particle clustering and particle ignition. A pseudo-spectral reacting multiphase flow solver using a low Mach number approximation is employed. It features an established point-particle sub-model for reacting iron particles and describes the mass, heat and momentum transfer across the particle boundary layers in a two-way coupled regime. Within this setup, we perform a series of simulations covering a broad range of Reynolds and Stokes numbers. The results confirm the well-known observation from incompressible dispersed multiphase flows that particle clustering is most pronounced for particle clouds with Stokes numbers of order one. Here, strong particle clustering additionally facilitates the earlier ignition of heterogeneously burning iron particles. This is because groups of neighboring particles locally deposit more heat per volume than a single isolated particle, thus more strongly increasing the local fluid temperature and shifting particle reactions from kinetically-limited to diffusion-limited particle conversion. A Voronoi tessellation analysis shows that the particles packed together in clusters tend to ignite first, even for small Stokes numbers, at which clustering is less prominent. An increase in Reynolds number reduces the ignition delay times, especially for high Stokes numbers, since particles with higher inertia are more sensitive to the larger scales of the turbulent motion. The present findings are of practical importance for the design and flame stabilization mechanisms in future iron combustion burners.

Insights into the flame transitions and flame stabilization mechanisms in a freely falling burning droplet encountering a co-flow

The present study investigates the flame dynamics of a contactless burning fuel droplet under free fall subjected to a co-flow. The dynamic external relative flow established due to co-flow and droplet acceleration results in a series of droplet flame transitions. Different flame structures were observed, including a wake flame, reversed wake flame and enveloped flame. Following ignition, the droplet is allowed to fall through the central tube of a co-flow arrangement, and, at its exit, the droplet flame encounters the co-flow. The wake flame, which was established based on the droplet's instantaneous velocity of descent, encounters the abrupt relative velocity jump due to the co-flow. This causes the droplet flame to go through various transitions as it approaches equilibrium with the surrounding flow. Once it equilibrates, the droplet flame evolves in response to the instantaneous relative flow velocity. The droplet flame evolves by altering both its shape and the stabilization mechanism. Two stabilization mechanisms were identified for the droplet wake flame: edge-flame stabilization and bluff-body stabilization. The stabilization mechanism for different flame structures and the transition events have been theoretically analysed, and the relation between flame shape evolution and flow velocity has been determined based on the flow-field characteristics at the corresponding Re (Reynolds number) range. Furthermore, these correlations are employed in a mathematical formulation based on the spring–mass analogy, which predicts the droplet flame evolution after encountering the co-flow, including all the transition events.

Numerical investigation on flame-stabilization mechanism of supersonic-combustor-based high-temperature CO/H2 jet

A large-eddy simulation (LES) was performed on two different supersonic combustors based on a high-temperature CO/H2 jet, and the flame-stabilization mechanism was analyzed. The results showed that for a strut-cavity-based combustor with the high-temperature jet turned on, the jet ignited, and the strut promoted the mixing of the supersonic airflow and fuel. The cavity recirculation zone realized the exchange of energy and momentum between the high-temperature gas and fuel in the shear layer, resulting in a decrease in the chemical timescale—referred to as the flame-stabilization mode of the high-temperature jet and cavity shear-layer ignition. Moreover, in a strut-cavity-based combustor with the high-temperature jet turned off, the strut promoted the mixing process, and the cavity improved the residence time of the fuel, resulting in the flow timescale being larger than that of the reaction—referred to as the flame-stabilization mode of the strut-cavity recirculation-zone flame stabilization. In the strut-based combustor with the high-temperature jet turned on, the rocket jet performed an ignition role in the flame stabilization process— referred to as the flame-stabilization mode for high-temperature jet ignition. When the jet was turned off, the chemical timescale increased rapidly and continued to be larger than the flow timescale, resulting in flameout.

Effect of dimethyl ether blending on methane flame morphology transformation and stabilization mechanism in a micro-planar combustor

This paper presented a novel blended combustion solution for mixing methane/dimethyl ether/air in a micro combustion chamber. Simulations were used to compare the pure methane combustion with the blended combustion, focusing on the OH species distribution, y-axis velocity, and strain rate to reveal the flame dynamics differences in different combustion conditions. The results showed that the blowout limit was enhanced from 0.53 m/s to 1.18 m/s at a blending ratio of 50 %. The blending of DME was also beneficial in weakening the flame asymmetry, delaying the generation of inclined flames in the original combustion, and replacing the inclined flames by a double-peaked U-shaped flame and an inverted U-shaped flame at an equivalence ratio of 0.9. Overall, this study not only provides a reliable fuel mixture for enhancing the combustion process under microscale conditions, but also reveals the flame transition characteristics.

Numerical study on the ignition and combustion of a CH4 jet flame in diluted and undiluted coflows

This study numerically investigates the ignition and evolution of CH4 jet flames in a hot coflow (JHC) using large eddy simulation (LES). Three coflow oxygen fractions are carefully selected to compare moderate or intense low-oxygen dilution (MILD, YO2,C = 6 % and 9 %) and high temperature combustion (HTC, YO2,C = 23.3 %). Systematic investigations are performed on flame ignition, lift-off, reaction progress, and stabilization mechanisms. The results show that for the MILD cases, both the ignition process and final stable flame are controlled by turbulent mixing and chemistry in a state far from chemical equilibrium. In contrast, the reactions are much stronger for HTC, and the final flame is controlled by chemistry through equilibrium reactions. Moreover, for all three cases, the reactions always progress more rapidly at the lean side than at the rich side; thus, autoignition dominates the former, whereas flame propagation is more important for the latter. In addition, the premixing of fuel and oxidant prior to main reactions is crucial for realizing MILD combustion; therefore, the flame is lifted and stabilized by the autoignition of partially premixed reactants in the fuel-lean region. In contrast, the final HTC flame attaches to the burner exit and is stabilized by both the lean-side autoignition and rich-side flame propagation. For all three cases, the lean-side and stoichiometric flames adopt the nonpremixed mode, whereas the rich-side reactions are in the premixed mode, except for the flame base.

Research on influence of oxygen on flow/combustion characteristics in a scramjet combustor equipped with an O2-pilot strut

With the help of an O2-pilot strut, the stable supersonic combustion was achieved in a scramjet combustor fueled by liquid kerosene. Objective of this work is to investigate the effect of different mass flow rate of oxygen on the cold flow and combustion characteristics to explain the role of the supplied oxygen in supersonic combustion. A series of numerical simulations and experiments were carried out under the high inflow enthalpy condition (Ma = 2.8). High-speed photography and pressure measurement were used to record the flame images and pressure of combustor. A skeleton kerosene mechanism was used for combustion simulation for more details of combustion. Simulation results indicated that by adjusting the mass flow rate of oxygen, the severe rich-kerosene zone around the ignitor could be improved. Under the low mass flow rate of oxygen condition, limited by over-high equivalence ratio the ignition delay time was too long and heat release was too small to establish the global flame. On the contrary, under the high mass flow rate of oxygen condition, limited by over-low equivalence ratio, only little kerosene could be ignited with most of the kerosene carried downstream by high-speed flow resulting in a sharp increase in flame lift distance. The boundary of mass flow rate of supplied oxygen for combustion stabilization was obtained. The results in this paper would be valuable for reveling the ignition and flame stabilization mechanism in supersonic airflow.

Fuel/air mixing characteristics of a Micromix burner for hydrogen-rich gas turbine

As a promising hydrogen-rich gas turbine combustion technology, Micro-mixing combustion may produce a new flame stabilization mechanism due to the novel structure. To investigate the effects of nozzle structural parameters and inlet aerodynamic parameters on mixing characteristics, this paper firstly used the background oriented schlieren technique combined with the self-development program to obtain the mixing quality, and the program was based on the optical flow method and Gladstone-Dale equation. The fuel-air mixture process was predicted by three mass diffusion models, and the Kinetic Theory model matched the experimental results best compared with the Constant Dilute Approximation model and Dilute Approximation model. The results showed that the main mixing zone moved closer to the outer wall when the premixed channel diameter increased. And the vorticity decreased with the increased premixed length, leading to a decline in mixing efficiency. For inlet aerodynamic parameters, changes in equivalence ratios only affected the shear-layer vorticity. As the combustible component of syngas was in the range of 60%–40% (by vol.), the outer wall of the premixed channel could promote fuel-air mixture. Moreover, when the equivalence ratio ranged in 0.335–0.685, the heating value of fuel varied as 4.686–7.029 and 11.715 MJ/Nm3, fuel and air were premixed uniformly near the nozzle outlet.

Velocity nonuniformity and wall heat loss coupling effect on supersonic mixing layer flames

With urgent demand for reliable flame stabilization in air-breathing high-speed aircrafts' combustors under a wide range of operational scenarios, understanding flame characteristics under various inflow and wall thermal conditions is crucial for mixing layer-stabilized flames. In this paper, the effect of inflow velocity nonuniformity, wall heat loss and their coupling on flame structures, flame lift-off and stabilization mechanisms in the supersonic mixing layer are analyzed. An approach based on the Beta distribution model is innovatively proposed to generate the pseudo nonuniform inflow velocity profiles. Different flame modes, the regime diagram and key controlling parameters are quantitatively analyzed via the active subspace method. The results show that the coupling of velocity nonuniformity and wall heat loss can lead to three flame modes, i.e., the heat loss-dominant flame, the intermediate flame, and the nonuniformity-dominant flame, as the flame stabilizes from upstream to downstream regions with the normalized flame lift-off length Lnorm in the range of [0,0.50], (0.50,0.58], and (0.58,1.0], respectively. This finding is consistent with the autoignition flame stabilization mechanism. In addition, the fuel burnt-out ratio of the flame is found to be nonlinearly correlated with flame lift-off length and peaks approximately with Lnorm=0.8, owing to the subtle interactions of mixing and chemical reactions in the mixing layer. The viscous heating in the boundary layer may have a pronounced impact on the flame lift-off depending on the level of wall heat loss, and the predicted Lnorm variation could be approximately 26 percent of the combustor length with/without viscous effect under the isothermal wall condition.

Air-blast atomization and ignition of a kerosene spray in hot vitiated crossflow

Increasingly stringent regulations of pollutant emissions from aviation require rapid implementation of novel combustion technologies. Promising concepts based on moderate or intense low-oxygen dilution (MILD) combustion have been investigated in academia and industry. This MILD regime can be obtained from the recirculation of the hot vitiated combustion products to raise the temperature of the reactants, resulting in distributed reaction regions and lower flame temperatures. In the present work, we consider the air-blast atomization of a kerosene spray in crossflow, which enables efficient mixing between fuel and oxidizer. We investigate experimentally and numerically the effect of the spray air-to-liquid mass-flow ratio (ALR) variation on the reaction front and flame topology of a kerosene spray flame. The spray is injected transversely into a turbulent vitiated crossflow composed of the products of a lean CH4-H2 flame. The spray flame thermal power is varied between 2.5 and 5 kW, along with the atomizer ALR between 2 and 6. The experimental characterization of the reaction zone is performed using OH* chemiluminescence and OH and fuel planar laser-induced fluorescence (PLIF). The Large Eddy Simulations (LES) of the multiphase reactive flow provide good agreement with the experimental observations. Experiments and simulations show that the ALR governs mixing, resulting in different flame stabilization mechanisms and combustion regimes. Low ALR results in a relatively small jet-to-crossflow momentum ratio and a large spray Sauter mean diameter (SMD). A thick windward reaction region is formed due to inefficient shear layer mixing between the fuel spray and the crossflow. Meanwhile, the correspondingly large spray SMD leads to isolated penetration and localized combustion of fuel clusters. At high ALR, the higher penetration and the faster droplet evaporation due to the lower spray SMD result in an efficient entrainment-induced mixing between the two streams, forming more distributed reaction regions.

Combustion Characteristics of a Novel Ammonia Combustor equipped with Stratified Injection for Low Emissions

Ammonia – a carbon-free fuel- is being considered by various research groups and organizations as a potential alternative fuel to tackle some global warming issues of the 21st century. However, ammonia faces some challenges as a fuel, particularly, the production of unwanted NOx and unburned ammonia traces at the back end of combustion systems accompanied by unstable flame regimes. Therefore, new concepts are currently being developed to address these challenges whilst also providing stable, high-power operation. This work presents a newly designed stratified combustion system for gas turbines capable of operating with both premixed and stratified modes. The system has been initially evaluated using these two modes employing solely hydrogen and ammonia. Increase of hydrogen stratification resulted in the reduction of NO and NO2, while N2O slightly increased due to the reduction of fuel in the premixed system. Further tests were conducted to evaluate the potential of the systems to replace fossil fuels. Previous work done using methane as the fuel-to-be-replaced has shown interesting trends in terms of emissions and stability. Thus, to progress on the subject, the work conducted here attempted propane/ammonia/hydrogen using this innovative strategy. Results denoted how stratification has a strong impact on NOx production at the flame front, whilst the mechanism of flame stabilization of the new burner enables large operability regions under a great variety of fuel combinations. Strategic injection setups were identified, whilst the best propane/ammonia/hydrogen blends for stability and emissions reduction are also detailed for further implementation in larger systems.

Experimental investigation on ignition and cross-flame performance of dual-annular combustor

The ignition and co-flame process of the dual annular combustor(DAC) was studied using high speed camera in order to investigate the mechanism of flame stabilization and propagation in the combustor. Particle Laser Velocimetry (PIV) combined with high speed camera were used to measure the key properties of ignition kernels, including orientation, growth direction, movement velocity, flame propagation speed. Results showed the flow field structure obtained by PIV includes the primary air jet, the dilution air jet, the inner/outer recirculation zones, the central body jet, and the middle area of the central body. The ignition process could be categorized into the four stages: kernel generation at the end of the igniter, kernel movement at the recirculation zone, kernel growth at the head, and burner-scale flame establishment at the combustor, the flame had significantly different performance in each stage. The time of each stage decreases with the increase in the inlet Reynolds number, which significantly reduced the ignition delay time. Additionally, as the excess air coefficient increased, the ignition of pilot stage become more complex and the ignition delay and the cross-flame time increased. The relative location of the flame center and the centroid confidence interval of the flame path was used to judge ignition success. The flame path analysis showed that the transverse propagation mode of flame in the head is the determining stage of ignition. The splitting direction and the flame statement were related to the flame's position and shape in the flow field, the transverse flame splitting played a vital role in the ignition process. Two typical co-flame mechanisms existed in the cross-flame process of the DAC as follows: the flame tongue co-flame mechanism under small working conditions and the heat reflux combined small flame co-flame mechanism under large working conditions.

Intermittent flame lengths of buoyant jet flames in reduced and normal pressures: Experiments and theoretical analysis

Quantifying the intermittent flame length scale of buoyant jets is valuable for the understanding of the physical mechanisms of flame stabilization, risk assessment of fuel leakage fire in storage facilities, as well as the practical design of effective industrial burners. In this work, experimental studies are conducted to study the intermittent flame lengths in both reduced (0.64 atm) and normal (1.0 atm) pressures. The intermittent flame lengths are quantified by flame probability contours of propane flame using five nozzles with different diameters ranging from 4 to 10 mm with different heat release rates ranging from 4.31 kW to 49.52 kW. The intermittent flame lengths are found to be positively related to the heat release rate Q˙ and nozzle diameter d. Besides, results show that lower ambient pressure can bring about significantly higher intermittent flame lengths as compared with the results obtained in normal pressure. Moreover, based on physical analysis of the flow character of unburned fuel pressure dependence of flame length scale, a general model coupling with Froude number and nozzle diameter (d) and pressure effect (P∞/1.0atm)2/3 is established as Δlf/d(P∞/1.0atm)2/3=C1Frm−0.4(2−C2Frm−0.6) for the quantification of the intermittent flame lengths in both pressures. And the correlating results show that all the data in both pressures and those from previous literature can be well normalized with a satisfactory agreement. The current work can provide some supplemental knowledge on flame instability in the reduced pressure environment, a scientific evaluation model of fire length scale in fire safety strategies in fuel storage facility leakage cases, and good data reference for CFD validation.